Synthetic paper having microvoids in the inside which is obtained by stretching a thermoplastic resin sheet containing calcium carbonate powder, as disclosed in JP-B-60-36173 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-B-1-56091, U.S. Pat. Nos. 4,341,880 and 5,204,188, and JP-A-62-227933 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), is superior to pulp paper in water resistance and strength and therefore useful as outdoor posters, stickers for vending machines, labels of detergent bottles or frozen foods, and lately as paper for books that one can read while taking a bath.
Compared with pulp paper, the void-whitened synthetic paper using calcium carbonate for void formation on stretching is disadvantageous in that printing inks applied thereto have poor drying properties because the matrix thereof contains polyolefins having no polar group, such as polypropylene and polyethylene.
In addition, calcium carbonate particles fall off during offset printing and enter the printing ink via the rubber blanket. As a result, the ink fluidity is reduced and fails to be transferred smoothly from the printing machine to printing paper, resulting in poor print quality, as pointed out in JP-A-56-137960. This phenomenon is hereinafter called paper dust trouble.
To overcome these problems, coated paper prepared by coating the polyolefin-based synthetic paper with a pigment coating material usually comprising a filler, e.g., clay or silica, and a binder, e.g., polyvinyl alcohol or a styrene-butadiene latex, has been supplied. However, application of a pigment coat for improvement of ink drying properties means involvement of an extra step, which is not economical.